This invention is directed in general to data transmission systems. More specifically, the invention is directed to data transmission systems of the type including a data processing unit associated with a base radio station and one or more mobile or portable data terminals each associated with a mobile or portable radio by which digital data is transmitted among the various data terminals over at least one radio channel.
Referring to FIG. 1, a typical radio data transmission system includes a base radio station 10 controlled by or connected to a data processing unit 12. The base station and data processsing unit communicate with one or more mobile or portable stations 14 over a radio channel. The radio link may include a repeater 16 and may be either simplex or duplex. In systems including a plurality of mobile or portable stations wherein each has the capability of accessing a shared communication channel, it is necessary to prevent contention on the radio channel. To prevent contention, mobile or portable stations generally include a logic device for generating a channel busy signal. This channel busy signal is a function of some signal received on the communication channel. Generally, the channel busy signal is generated whenever a receiver, associated with a mobile or portable data terminal, detects any signal on the radio channel. The effect of the channel busy signal is to inhibit data transmission by the data terminal detecting such signal so that it will not interfere with another data terminal already using the channel. Data transmission systems may, in addition to transmitting digital data, provide for the transmission of voice traffic.
When operating known radio data transmission systems, the presence of long-term interference (spurious signals) on a radio channel causes a serious operational difficulty. The logic used in known data transmission systems generates a channel busy signal which does not adequately discriminate between legitimate data or voice transmission on the channel and an interfering spurious signal. Thus, very often data transmission is inhibited when a data terminal detects a spurious signal (noise) on the channel. Although it is appropriate for a particular data terminal to have its transmission inhibited when another data terminal is transmitting a data or voice signal on the channel, it is not desirable to inhibit transmission merely when there is noise on the channel. Inhibiting data transmission each time there is noise on the radio channel causes a significant down time for a data terminal and subsequent loss of throughput. Using conventional logic, a data terminal is usable only as long as no radio interference is received by the repeater, the base station or the mobile or portable data terminal itself. If either the base station or repeater receives a spurious interfering signal that is interpreted as a valid RF signal for that receiver, a carrier activity signal (known more particularly as a Carrier Activated Switch or CAS signal) is generated by the receiver associated therewith indicating that the radio channel is busy. A CAS signal alone is sufficient to generate, according to the conventional logic, a busy signal. A data terminal seeing this busy signal recognizes the channel as being busy when in fact it is not busy. Thus, data traffic is completely inhibited until the spurious signal is no longer received by either the base station or repeater.
Similarly, a particular mobile or portable data terminal can receive a spurious signal. Upon such reception, the mobile or portable unit receiving a spurious signal generates a false channel busy signal inhibiting its own transmission until the interfering signal leaves. Both of these cases present an undesirable situation.
In data transmission systems including voice transmission capability, a continuous tone coded squelch system (CTCSS) is generally used to unmute radio speakers for voice communication. A typical CTCSS arrangement is illustrated by U.S. Pat. No. 4,171,516--Challen et al., incorporated herein by reference. In voice systems already utilizing a CTCSS, it would be possible to add a second CTCSS to protect against noise. However, the addition of a second CTCSS to protect against noise would be quite costly. In addition, a second CTCSS would not be easily accommodated in most mobile radio units designed to accept one such CTCSS circuit.
Known techniques for addressing the false channel busy problem involve the use of any of several different receiver signals to generate the carrier activity signal (CAS) for inhibiting all other mobile or portable units on the radio channel when any one unit is transmitting. A CTCSS could be part of the specific circuit arrangement used to generate the CAS. However, if a CTCSS circuit is used as part of the logic busy function in this manner, a second CTCSS cannot be used conveniently for unmuting speakers since the radio does not accommodate two circuits. If no CTCSS circuit is used, interfering signals can "busy out" the system especially if there is a repeater in the system.